Fridlyand L, Kaplan A, Reinhold L
V.F. Kuprevich Institute of Experimental Botany, Belarus Academy of Sciences, Minsk.
Biosystems. 1996;37(3):229-38. doi: 10.1016/0303-2647(95)01561-2.
This paper assesses the contribution of a postulated CO2-scavenging system to the efficient operation of the CO2-concentrating mechanism (CCM) in cyanobacteria. A quantitative model for the CCM is presented which incorporates an energy-dependent carbonic anhydrase-like entity located at or near the inner surface of the plasma membrane. This entity, which converts CO2 to HCO3- against the thermodynamic potential, scavenges CO2 leaking outward from the carboxysomes, and, further, converts CO2 entering from the medium to HCO3-, thus maintaining an inward diffusion gradient along which CO2 enters passively. The model resembles our earlier models in postulating that CO2 and HCO3- are not at equilibrium throughout the greater part of the cell, and that CO2 is generated in high concentration at carbonic anhydrase sites within the carboxysomes. The model further takes into account the concentric thylakoid membranes which surround the carboxysomes, and events in the periplasmic space and the unstirred layer surrounding the cell. Implications of the predicted steady state fluxes of CO2 and HCO3-, and of their steady state concentrations in various cellular compartments, are discussed. The plasma membrane carbonic anhydrase-like activity lowers the photosynthetic Km for external Ci, as well as decreasing the inorganic C 'leak', but it may not save on energy expenditure.
本文评估了一种假定的二氧化碳清除系统对蓝细菌中二氧化碳浓缩机制（CCM）高效运行的贡献。提出了一个CCM的定量模型，该模型纳入了位于质膜内表面或其附近的一种能量依赖型碳酸酐酶样实体。该实体逆着热力学势将二氧化碳转化为碳酸氢根，清除从羧酶体向外泄漏的二氧化碳，并且进一步将从培养基进入的二氧化碳转化为碳酸氢根，从而维持一个内向扩散梯度，二氧化碳沿着该梯度被动进入。该模型与我们早期的模型类似，假定在细胞的大部分区域二氧化碳和碳酸氢根并非处于平衡状态，并且在羧酶体内部的碳酸酐酶位点会产生高浓度的二氧化碳。该模型还进一步考虑了围绕羧酶体的同心类囊体膜，以及周质空间和细胞周围的静止层中的情况。讨论了预测的二氧化碳和碳酸氢根稳态通量及其在各种细胞区室中的稳态浓度的影响。质膜碳酸酐酶样活性降低了对外部无机碳的光合米氏常数，同时也减少了无机碳的“泄漏”，但它可能无法节省能量消耗。
